1. Field of the Invention
The present invention relates to techniques for manufacturing components comprising shape memory alloys and other metals, and more particularly to soldering techniques and flux materials used in soldering.
2. Description of Related Art
In recent years, nickel-titanium shape memory alloys have found an increasing number of applications in medical devices where their unique properties permit the design of miniature structures for special purposes within arteries and elsewhere within the body. As described in U.S. Pat. No. 6,371,970, tiny nickel-titanium wire hoop forms may be tightly folded and then opened within an artery in order to act as the framework for a tiny net designed to capture plaque fragments dislodged from the artery wall into the blood stream during angioplasty procedures and thus prevent potential embolism.
In another type of application, described in U.S. Pat. No. 6,090,105, curved nickel-titanium memory alloy wires can be retracted within a straight trochar and, when in the body, extended to regain their pre-set curved form to act as electrodes for applying localized microwave energy to ablate tumors.
The novel flux of the present invention will add flexibility of design by permitting the joining of nickel-titanium alloys to other metal structural components of medical devices such as stainless steel.
The oxidation resistance of refractory metals may be attributed to the presence of naturally formed protective surface oxides (or oxygen containing compounds) which, when dense and adherent, shield the underlying metal from further oxidation. This quality is desirable in alloys designated for high temperature use. However, without mechanical removal or chemical dissolution by an applied liquid phase flux, the oxide skin also prevents the wetting of the refractory metal by low melting solder.
Acid-containing fluxes are frequently used to remove the oxide layer on various metals to promote wetting by low temperature (below 300° C.) solders such as conventional lead-tin mixtures, tin with 0 to 6% silver, and 80% gold-20% tin alloy.
For higher temperature joining of metals by brazing, surfaces are first cleaned of oxide by applying and heating a flux typically containing borate and fluoride salts. In brazing, the liquid flux permits molten filler metals such as copper-silver alloys to wet and flow on the surfaces of the parts being joined. In general, a problem arises when the temperature needed for proper fluxing is greater than a limiting temperature associated with preserving the mechanical properties of the metals involved.
The natural oxide formed on nickel-titanium alloys having unique shape memory or superelastic behavior (Nitinol, Elastinite) is not easily wetted by low temperature solders such as 80% gold-20% tin or 96% tin-4% silver. Accordingly, soldering to shape-memory alloys such as Nitinol presents special requirements. The melting temperature of the solder and the temperature for good fluxing action must be less than any annealing temperature previously applied to establish a desired shape for a device designed to utilize the unique memory properties of the nickel-titanium alloy. Certain acidic aqueous fluxes containing phosphoric acid satisfy the low temperature requirement but are known to cause embrittlement of the nickel-titanium alloy, presumably due to hydrogen produced by a cathodic reaction on the metal surface. Hydrogen embrittlement caused by heated phosphoric acid flux has been described by Pelton et al. (pages 395-400, Proceedings of the Second International Conference on Shape Memory and Superelastic Technologies, March 1997; published by SMST, Santa Clara Calif., ISBN 0-9660508-1-9).
In the present invention, the molten hydroxide eutectic is non-aqueous, so that free hydrogen ions are not available to act as a source for hydrogen embrittlement through electrochemical reaction.
Other fluxes clean Nitinol but require temperatures that can produce a loss or diminution of the desirable mechanical properties of the shape memory alloy. T. Hall, in U.S. Pat. No. 5,242,759 and U.S. Pat. No. 5,354,623, teaches a flux for soldering nickel-titanium alloys which is an aqueous paste mixture of organic amines, hydrofluoric or hydrochloric acid and various chloride salts wherein the mixture only becomes active at temperatures greater than 246° C. (475° F.).
Nanis et al. (U.S. Pat. No. 5,695,111) find that the cleaning of oxide from the surface of nickel-titanium alloy can be accomplished at temperatures as low as 170° C., the melting temperature of a sodium hydroxide-potassium hydroxide eutectic mixture. Nanis et al. teach a two-layer method in which a layer of liquid hydroxide flux is maintained over a layer of liquid 80% gold-20% tin solder alloy, which melts at 185° C. A part is first immersed in the flux layer for a specified time for oxide removal and is then more deeply immersed so as to make contact with the underlying layer of liquid gold-tin alloy solder. The gold-tin solder wets and coats the freshly cleaned nickel-titanium. After the part is withdrawn from the two-layer array, the solidified gold-tin solder layer provides an intermediary surface which is then wettable by other solder compositions, such as 95% tin-5% silver, for assembly of the nickel-titanium part in a device.
Another method for promoting adhesion of tin-silver solder is to electroplate the nickel-titanium surface with a metal known to be wettable by liquid solder. It is desirable that such plated metals have either no oxide layer of their own, or can be cleaned with conventional flux during soldering. Electroplated gold, nickel and other plated metals serve the purpose. However, as experienced in most plating systems, oxide removal and preliminary cleaning of the object to be plated is accomplished by a series of immersions in various etchants such as mixtures containing hydrofluoric and other acids. Hydrogen embrittlement is possible from such etch treatments.
Good plating practice requires that the part be transferred quickly into the plating tank after cleaning and rinsing steps in order to prevent re-oxidation. Thus, the burden falls on the plater to obtain good adhesion of the plated layer to the nickel-titanium substrate. The extra step of plating a solder-wettable layer adds to the expense and complexity of manufacture, particularly for small medical device parts which require special fixturing to assure good current distribution and electrical contact during plating.
Problems with the plating of nickel on nickel-titanium alloys have also been noted by P. Hall (“Methods of Promoting Solder Wetting on Nitinol”, page 126, Proceedings of the Second International Conference on Shape Memory and Superelastic Technologies, March 1997; published by SMST, Santa Clara Calif. ISBN 0-9660508-1-9).